This invention generally relates to the imaging of moving ultrasound scatterers. In particular, the invention relates to methods for positioning the gate or sample volume (hereinafter xe2x80x9csample gatexe2x80x9d) in medical diagnostic ultrasound imaging.
Premium medical diagnostic ultrasound imaging systems require a comprehensive set of imaging modes. These are the major imaging modes used in clinical diagnosis and include timeline Doppler, color flow Doppler, B mode and M mode. In the B mode, such ultrasound imaging systems create two-dimensional images of tissue in which the brightness of a pixel is based on the intensity of the echo return. Alternatively, in a color flow imaging mode, the movement of fluid (e.g., blood) or tissue can be imaged. Measurement of blood flow in the heart and vessels using the Doppler effect is well known. The phase shift of backscattered ultrasound waves may be used to measure the velocity of the backscatterers from tissue or blood. The Doppler shift may be displayed using different colors to represent speed and direction of flow. In the spectral Doppler imaging mode, the power spectrum of these Doppler frequency shifts are computed for visual display as velocity-time waveforms.
One of the primary advantages of Doppler ultrasound is that it can provide noninvasive and quantitative measurements of blood flow in vessels. Given the angle between the insonifying beam and the flow axis (hereinafter referred to as the xe2x80x9cDoppler anglexe2x80x9d), the magnitude of the velocity vector can be determined by the standard Doppler equation
xe2x80x83xcexd=cf4/(2f0 cos xcex8xe2x80x83xe2x80x83(1)
here c is the speed of sound in blood, fo is the transmit frequency and fd is the motion-induced Doppler frequency shift in the backscattered ultrasound signal.
In conventional ultrasound spectral Doppler imaging, the operator is required to manually position the sample gate to the measurement location in a two-dimensional image with or without color flow data. The operator also needs to manually adjust the sample gate size relative to the diameter of the vessel to be studied. From the acoustic data acquired over many transmit firings, Doppler frequency spectral data is obtained via standard Fast Fourier Transform (FFT) spectral analysis.
A two-dimensional B-mode or color (velocity or power) flow image can be used to guide the positioning of the pulsed Doppler sample gate (volume) for blood flow spectral analysis. Color flow is usually used for imaging smaller vessels since it provides a more sensitive detection of weak flow signals. Regardless of whether B-mode or color flow image data is used, it is desirable to be able to maintain the Doppler sample gate at the selected vessel position for, e.g., 10 sec to enable reliable diagnostic waveform index (e.g., systolic to diastolic ratio) calculations.
In practice, however, it is often difficult to keep the sample gate on the vessel of interest due to probe and/or patient motion, including cardiac and breathing. To minimize the associated artifacts or data dropouts, the patient is often asked to hold his or her breath for a number of seconds, which could be difficult for some elderly or sick patients. The sonographer may also try to track vessel motion manually by moving the probe. It should be noted that if color flow imaging is active, the user usually wants to track the moving colorized vessel and not the background B-mode image anatomy. But in practice, it can still be challenging to obtain a good pulsed Doppler sampling of the colorized blood vessels in organs like the kidney because of probe or patient motion.
In spectral Doppler techniques, the angle between the Doppler beam cursor (beam centerline) and the blood vessel orientation (i.e., slope) cursor (i.e., the Doppler angle) is used to convert Doppler frequency shift into velocity units according to Eq. (1). If the Doppler angle changes due to vessel movements, it needs to be updated for correct velocity calculation.
U.S. Pat. No. 5,365,929 describes the use of multiple range gates and multiple Doppler beams to scan a region of interest. By comparing some signal characteristic, such as total power or maximum velocity, of the multiple sample volumes, the scanner automatically selects the best sample gate for full spectral analysis and display. It will appear to the user that the scanner has automatically positioned the sample gate at a location where the Doppler signal is optimal in some sense. The main difficulty with this approach, however, lies in the definition of the signal characteristic for ranking the multiple sample volumes. The obvious choices are signal power or velocity, but, for example, it is quite possible that the user may want to study a diseased portion of the vessel which generates neither the strongest nor the highest velocity signal. Also, in duplex Doppler exams, color data is not available.
European Patent Application No. 0 842 638 A2 describes a method of tracking vessel walls in the B-mode image, and then automatically adjusting the sample volume size to ensure the entire vessel diameter is always covered for volume flow estimation. The vessel wall tracking is based on edge detection algorithms which may work well if the vessel is relatively large with clearly defined walls, and provided that the center of the sample volume remains inside the two walls from frame to frame.
European Patent Application No. 0 985 380 A1 describes a method for automatic positioning of the Doppler sample gate based on bloodstream or color flow information. Among various specific applications, this method can be used to automatically set the sample gate cursor at an optimal position when the sample gate is first brought up in the image, or when it is being moved. The optimal position may be defined by a color flow pixel showing the highest velocity, or the center point of the largest flow segment, or the center point of the next best flow segment, etc. In other words, this invention pertains to selection of an optimal Doppler sampling location within a given color flow image, and not tracking any given vessel from frame to frame.
An automatic method of locking the pulsed Doppler sample gate onto a moving vessel, and updating the Doppler angle when necessary, would clearly facilitate Doppler blood flow studies and/or improve the speed of examination.
The present invention is directed to a method and an apparatus which automatically keeps the Doppler sample gate at a pre-selected vessel position in B-mode or color flow images during tissue or probe motion. The goal is to lock the sample gate onto the selected vessel automatically when the vessel position has changed. Optionally, the vessel slope cursor is automatically updated when the vessel position has changed. The method employs pattern matching of images from successive frames to determine how much a vessel in the image frame has been translated and rotated from one frame to the next.
In accordance with one preferred embodiment of the invention, a cross-correlation method applied to the imaging data in the spatial domain is used to determine the relative object translation and/or rotation between image frames. In accordance with another preferred embodiment, a matched filtering method is applied to the imaging data in the frequency (i.e., Fourier) domain to determine the relative object translation and/or rotation between image frames. In accordance with a further preferred embodiment, the image registration is performed by combining the spatial and frequency domain methods, e.g., by performing the scaling and rotation registration first using one method and then using the other method to find the x-y translation offsets.
Being a pattern matching technique, the method will work well for B-mode image data that shows clear tissue structures, B-mode flow images, and color flow images. The use of color flow is preferred for smaller vessels (e.g., in the abdomen) which do not show up well in the B-mode image. Another advantage of using color flow is that it can be used directly for vessel segmentation to provide a binary (flow or no flow) image for pattern matching operations.